1. Field of the Invention
The present invention relates to a circuit board structure for a low noise block down-converter, and more particularly, to a circuit board structure capable of transmitting two radio-frequency signals across each other.
2. Description of the Prior Art
A satellite communication receiver may include a dish reflector and an LNBF (Low Noise Block Down-converter with Feedhorn). The LNBF is used for gathering satellite signals reflected by the dish reflector and converting the satellite signals into intermediate signals, and then transmitting the intermediate signals to a backend satellite signal processor for signal processing, thereby enabling the playing of satellite television programs.
Please refer to FIG. 1, which is a structural circuit diagram of a conventional LNB (Low Noise Block down-converter) 10. The LNB 10 has a function of outputting dual signals for two users. The LNB 10 includes LNAs (Low Noise Amplifiers) 101-112, power dividers 121-124, filters 131 and 132, mixers 141 and 142, oscillators 151-154 and a cross structure 160. Connection relations between the elements comprised in the LNB 10 are shown in FIG. 1.
In operation, when the satellite signals are received by the LNB 10, the satellite signals may be separated into an RF (Radio-Frequency) signal SV and an RF signal SH according to different polarizations, wherein the RF signal SV is vertically polarized and the RF signal SH is horizontally polarized. Operating voltages of the LNB 10 may be switched to control the elements comprised in the LNB 10 to perform signal processing on the RF signals SV and SH. The operating voltages for respectively processing the RF signals SV and SH are 13 volts and 18 volts. As the RF signal SV entered the LNB 10, the RF signal SV may be amplified by the LNAs 101 and 102 for two levels of signal amplification first, power divided by the power divider 121, and then part of the RF signal SV is amplified by the LNA 103 and the rest of RF signal SV is transmitted to the LNA 109 to be amplified by the LNA 109. Output terminals of the LNAs 103 and 104 may be coupled together to synthesize the RF signals SV and SH into a synthesized RF signal SVH1, the RF signal SVH1 may be amplified by the LNA 105, filtered by the filter 131, and mixed with a local oscillate signal L1 or L2 by the mixer 141, so that the RF signal SVH1 may be down converted into an IF (Intermediate Frequency) signal S1.
Likewise, as the RF signal SH enters the LNB 10, the RF signal SH may be amplified by the LNAs 107 and 108 for two levels of signal amplification first, power divided by the power divider 123, and then part of the RF signal SH is amplified by the LNA 110 and the rest of RF signal SH is transmitted to the LNA 104 to be amplified by the LNA 104. Output terminals of the LNAs 109 and 110 may be coupled together to synthesize the RF signals SV and SH into a synthesized RF signal SVH2, the RF signal SVH2 may be amplified by the LNA 111, filtered by the filter 132, mixed with a local oscillating signal L1 or L2 by the mixer 142, so that the RF signal SVH2 may be down converted into an IF signal S2.
In such a structure, the LNB 10 may control operations of the oscillators 151-154 to respectively generate the local oscillating signals L1 and L2. Or, the LNB 10 may further control the power dividers 122 and 124 to adjust signal intensities of the local oscillating signals L1 and L2, so as to generate the IF signals S1 and S2 having different operating frequencies. For example, the following equations are down-conversion formulas of the LNB 10 for a Ku operating band: (Unit:GHz)SV/SH(10.7−12.75)−L1(9.75)=S1(0.95−3.0)SV/SH(10.7−12.75)−L2(10.6)=S2(0.1−2.15)
Please refer to FIG. 2, which is an appearance diagram of the LNB 10. The LNB 10 includes circuit boards 11 and 12, spacers 13 and 14, a housing 15, output ports P1 and P2 and a plurality of thru pins 16. The circuit boards 11 and 12 are respectively disposed on two sides of the housing 15, the circuit boards 11 and 12 may be disposed with circuits or elements shown in FIG. 1 for performing signal process. The spacers 13 and 14 are respectively disposed on the circuit board 11 and 12 for covering the circuit boards 11 and 12. The thru pins 16 may penetrate through the circuit boards 11 and 12 and the housing 15 for transmitting signals flowing between the circuit boards 11 and 12. The output ports P1 and P2 are coupled to the circuit board 11 for respectively outputting the IF signals S1 and S2 to the backend satellite signal processor (not shown in FIG. 2).
However, since operating frequencies of the satellite signals, i.e. the RF signals SV and SH and the IF signals S1 and S2 are high, a return loss and an insertion loss of the RF signals SV and SH may be increased in the structure shown in FIG. 2. Specifically, a characteristic impedance of the thru pins 16 may be different from characteristic impedances of the circuit boards 11 and 12, and thus the RF signals SV and SH may flow across discontinuous impedances between the thru pins 16 and the circuit boards 11 and 12, which may increase the return loss and the insertion loss of the RF signals SV and SH.
Moreover, an isolation between any two thru pins 16 may be low, which may cause the RF signal flowing on the two thru pins 16 to interfere with each other by coupling or radiation, i.e. signal crosstalk. For example, except for the RF signals SV and SH, other signals such as the IF signals S1 and S2 and the local oscillating signals L1 and L2 may be viewed as a noise source and radiated by the thru pins 16 due to signal reflection or signal leak. In FIG. 1, assume the mixer 141 utilizes the local oscillating signal L2 generated by the oscillator 152 to mix with the RF signal SVH1. However, the local oscillating signal L1 generated by the oscillator 153 flows from the mixer 142, the filter 132, the LNAs 111 and 109 to the LNAs 104 and 105 at the cross structure 160 by coupling, and goes flowing to the filter 131 and finally the mixer 141. In such a situation, the IF signal S1 generated by the LNB 10 may include noises generated by mixing the local oscillating signal L1 with the local oscillating signal L2. The noise may be described as the following equation: (Unit:GHz)L1(10.6)−L2(9.75)=0.85
To eliminate the frequency 0.85 GHz and its harmonic frequency 1.7 GHz, an additional filter may be required or a change in the specification of the filter 131, which may increase a difficulty to design the LNB 10 and a production cost as well.
On the other hand, for a production process, it may take a lot of work or time to assemble the thru pins. Besides, two circuit boards and two spacers may increase a weight of the LNB 10, which not only increases the production cost and also increases a difficulty for installing a satellite television system. Therefore, there is a need to improve the prior art.